This invention relates to improvements in diffusers used with centrifugal compressors. Of particular concern are the diffusers and compressors used in gas turbine engines of small to medium size. In these engines, it is common practice to configure a high pressure ratio compressor which delivers a compressible fluid, usually air, at high velocity to a diffuser wherein the fluid is decelerated to produce a pressure rise.
Many different types of diffusers have been built over the last 50 years. U.S. Pat. No. 2,157,0002 is an example of an early diffuser. French Pat. No. 977,357 shows a double walled rotary diffuser wherein both side walls are supported on bearings mounted on the hollow shaft which drives the rotor. Friction transmitted via the bearing race helps to drive the French rotary diffuser. U.S. Pat. No. 3,460,748 discloses a compressor having disks which extend radially beyond the compressor blades to form a rotating vaneless diffuser passage which is followed by a vaned stationary diffuser. However, since the rotary vaneless section in U.S. Pat. No. 3,460,748 has walls which are integral with the blades of the compressor, they will be traveling at the same speed as the compressor. As a result, the periphery of the rotary diffuser section is traveling at high speed with respect to the stationary vaned second stage.
The present invention solves this problem by utilizing a rotating vaneless stage which turns at a fraction of the speed of the compressor rotor. Energy to turn the rotary stage comes from three sources. First, driving torque for the rotating diffuser element comes from bearing and oil film friction between the rotary element and the drive shaft of the compressor. Second, gas molecules striking the walls of the rotary diffuser section at an angle impart a torque couple force. Third, air striking the struts which connect one sidewall with the other imparts a driving force until the design speed of rotation is reached. As a result, a diffuser built according to the invention enables one to efficiently match a compressor rotor delivering gas at supersonic velocity to a stationary diffuser by introduction of a rotating stage having sidewalls which travel at a speed which is in a range that is approximately midway between that of the periphery of the impeller and the stationary vanes of the diffuser second stage.